Lecture 11 - Early Hominins

timeline

paleocene - first primates

eocene - adapoids/omomyoids

oligocene - first anthropoids (monkeys/apes)

miocene - first hominoids (apes)

pliocene - hominins (1st probable one in late miocene)

events

chimp/human split: 5-8 mya

did our most recent common ancestor have features that were similar to the living apes?

hominoids vs hominins

hominoids

• humans, recent ancestors, extant apes, extinct apes

hominin

• humans and recent ancestors (genus: ahelanthropus, Ardipithecus, Australopithecus, Homo)

5 derived traits of hominins (relative to extant apes)

1. obligate bipedalism

2. dentition and jaw musculature differ (parabolic arcade, small canines, etc.)

3. relatively large brains

4. slow development

5. material and symbolic culture; spoken language

important hominin traits visible in fossil record

• skeletal features for bipedalism

• cranial size

• canine size and general dentition

morphology: identifying bipedalism

• femur, tibia, patella shape

• pelvis shape - shorter, broad, and bowl-shaped

• foramen magnum - large opening at the base of the skull where the spinal cord connects to the brain, positioned relatively forward (more forward position helps the head balance over a vertically oriented spine, which is consistent with upright posture)

hominoid fossils in early miocene

• more common than monkey fossils in east africa

• found in areas that were once tropical forests

proconsul

• 1st hominoid (ape)?

• early miocene

• considered hominoid because it shares traits with modern apes

  • no tail

  • forward facing eyes

  • relatively smaller snout

  • relatively large brain

    • ancestral to chimp, gorilla, orangutan (and human?)

paleoenvironment and evolution

high climatic variability in past 7 million years

reconstructed through identifying fossils of plants/fauna, indicators of precipitation and temperature

what adaptation might have driven hominin evolution?

theories for bipedalism as an adaptation

1. thermoregulation: less surface area exposed to sun

2. energy conservation: bipedal walking/running is energetically efficient

3. freeing hands: facilitates carrying objects, tool use

savannah hypothesis

• shrinking forests in late miocene/early pliocene led early hominins to increase their time on ground

• thermoregulation

• BUT increased savannah area was not a consistent trend - more likely are adapted for variability (generalists)

Sahelanthropus tchadensis (Toumaï)

• (hope of life)

• 7-6mya

• found in chad (further west than other significant finds) by michele brunet and likius andossa

• date based on associated fauna

• brain: 320-350 cc

• smaller canines and thicker enamel than apes

• bipedal?

• where does it fit on ape/human family tree

debate over skull found in chad

brunet claims:

• hominin

• obligate biped

• ancestral to homo

milford wolpoff claims:

• hominoid (ape)

• facultative biped

• ancestral to apes or went extinct

S. Tchadensis is not only candidate for hominin ancestor in late miocene

Grooves in the femurs of O. tugenensis, presumably points where muscles and ligaments attached, suggest that the species was bipedal

orrorin tugenensis

“original man”

when: 6mya

where: kenya

morphology of

• hand phalanx - climbing adaptations retained

• teeth - small molars, thick enamel

• habitat: savannah

humans vs apes walking

humans: ADDucted hips, narrow steps, valgus knees, swing side pelvic drop

ape: ABducted hips, swing side pelvic elevation, varus knees, wide steps

oldest human femur controversy

senut claims

• bipedal

• bumps australopithecines off hominin line

tim white claims

• bipedal, but how?

• does not displace australopithecines

Orrorin tugenesis femoral morphology and evolution of hominin bipedalism

• O.t. femur differs from apes and homo

• most resembles australopithecus and paranthropus

• bipedal, but not closer to Homo than to australopithecus

• this hip morphology evolved early and persisted for ~4 m.y. until changes in late pliocene

ardipithecus ramidus

ardi = ground, ramid = root

• 5.8-4.4mya in Ethiopia discovered by Tim White

• cranium: 300-350 cc

• at least 36 individuals

• mosaic of ape and australopithecine traits

• ~4 feet tall

• locomotion? how do we know?

  • less specialized foot

  • rigid foot offered stabilization for heel and toe off

  • facultative biped; arboreal clamberer

  • not a knuckle walker

  • pelvis difference…

• woodland-forest habitat, NOT savannah

• teeth: reduced canines with little size difference between sexes → lower pressures on intra-sexual selection, minimal social aggression, no dietary specialization (generalist), wear patterns and thin enamel suggest diet was not abrasive

• about human origins: modern apes evolved social and locomotor behaviors IN PARALLEL (ex: knee walking is derived trait) - relatively long period of time which hominins were both adapted for arboreal quadrupedalism and terrestrial bipedalism

• our MRCA with chimps was more primitive than initially thought (probably had a more monkey like foot)

• bipedalism did not evolve in the savannah

commonalities

• adaptations for bipedalism

• earlier physical anthropologists believed increased brain size came first (NOT TRUE)

• among hominins, bipedalism is more ancestral trait, large brains more derived

phylogeny

• ardi and tchadensis have similar dentition and cranial capacity

• ardi has smaller dentition, less sexually dimorphic

• same genus????